Composition and method for coating zinc and zinc alloy sheets



United States Patent O COMPOSITION AND METHOD FOR COATING ZINC AND ZINC ALLOY SHEETS Harry J. Hartman, Baltimore, Md., assignor to Allied Research Products, Inc., Baltimore, Md., a corporation of Maryland No Drawing. Application April 22, 1955, Serial No. 503,327

Claims. (Cl. 1486.2)

This invention relates to the treatment of zinc or alloys thereof and is used for coating purposes on strip line operations such as mill, hot-galvanized or acid-zinc sheets.

The principal object of the invention is to impart a clear protective surface or film, having improved corrosion resistance, on zinc or zinc alloy used in strip line operation by subjecting the surface of the strip being treated to immersion in a filming bath.

Another and more specific object of the invention is to provide a clear protective film on a zinc or zinc alloy coating used for mill, hot-galvanized and acid-zinc sheets thus preventing discoloring, e. g., white bloom, which normally forms during storage by the highly corrosive action of the zinc or zinc alloy under moisture conditions, particularly when sheets are stacked one on another.

Another object of the invention is to provide a treating solution and method which will enable the above mentioned result's to be uniformly obtained on a commercial scale.

An additional object of my invention is to provide a composition for use in aqueous acidic solution to impart a clear protective coating to zinc and zinc alloy coated mill-galvanized sheets comprising at least one chromic compound selected from the group consisting of chromic acid and water soluble salts thereof, at least one boric compound selected from the group consisting of boric acid and water soluble salts thereof, at least one chloride containing compound selected from the group consisting of hydrochloric acid and water soluble salts thereof and a nitrate containing compound, selected from the group consisting of nitric acid and water soluble salts thereof all ingredients being of sufiicient strength and quantity to adjust the pH of the resulting solution to between about 0.2 and 4.0.

Still another object of my invention is to provide a composition as set forth heretofore wherein said chromic compound is in amount equivalent to from about 3 to 100 grams of sodium bichromate per liter of solution, the boric compound is an amount equivalent to from about 1 to 35 grams of boric acid per liter of solution, the water soluble chloride is in amount of 0.1 to 20 grams per liter of solution, and nitric acid in amount sufficient to maintain a pH of solution of about 0.2 to 4.0.

The following preferred formulation has been found to provide a clear colorless finish in actual strip line operation and to eliminate discoloring, e. g., white bloom, during normal storage and, in addition, impart desirable resistance to corrosion produced by a salt atmosphere and as observed by standard test on salt spray testing apparatus:

2,799,601 Patented July 16, 1957 The foregoing composition will promote passivation and corrosion resistance on mill, hot-galvanized and acidzinc sheets.

The three dry constituents of the foregoing example are dissolved in water, preferably lukewarm, and the nitric acid added until a pH of about 0.4 is attained. Then, if desired, the wetting agent mentioned or any suitable commercially available agent is added to avoid the possibility of water break on the surfaces to which the solution is applied.

The product of the above example is used successfully in amount of about 3 ounces per gallon of water, and within a range of about 2.5 to 20 ounces per gallon of water.

The sheet to be treated is sprayed or immersed in the solution having a temperature of about to 200 F. for 1 second to 30 seconds. The higher the temperature and longer the time of immersion, the greater the reaction and the more color imparted to the sheet. A good passivation temperature for the bath is 170 F. and the immersion time is 4 seconds. Also a good treatment is provided by immersing the sheet in a bath at about 70 F., room temperature, for 15 seconds. Both treatments produce satisfactory corrosion resistance and passivation.

The sodium bichromate plus the chloride activator produce a protective film when buifer supplied by the boric acid and the pH adjusted by the nitric acid are in correct balance.

The pH of the treating solution may vary between about .2 and 4.0, the solution of the foregoing example being preferably used at a pH of 1.4 or a pH maintained between about 1.4 and 1.5 for good passivation and when more color is desired, the pH is lowered, i. e., .2 to 1.4, which affords increased protection as indicated by the increased yellow or irridescent film.

In referring in the appended claims to zinc surfaces, 1 mean a surface to be passivated and rendered corrosion resistant having its entire area or a substantial area, of zinc or zinc alloy.

It is also tobe understood (1) that sodium bichromate may be substituted by other equivalent chromium compounds in equivalent amounts notably chromic acid and (2) that boric acid may be substituted by other equivalent boron containing compounds in equivalent amount notably sodium or potassium borate, and each within the ranges recited above. Similarly sodium chloride may be substituted by its equivalent hydrochloric acid, potassium chloride and other chlorides as recited above. Sodium nitrate may be substituted as an equivalent for nitric acid, both for the purpose described herein and in amounts to accomplish similar functions, within the ranges recited above.

The use of a wetting agent is preferred but is optional, any suitable agent being used.

I claim:

l. An aqueous acidic solution for passivating and im proving corrosion resistance of zinc surfaces comprising 3 to g./l. NazGrzOmZHzO; 1 to 35 g./l. HsBOs; 0.1 to 20 g./l. NaCl; and 1 to 10 ml./l. HNO (42 B.).

2. An aqueous acidic solution for passivating and improving corrosion resistance of zinc surfaces comprising 10 g./l. NazCrzOmZHzO; 10 g./l. H3BO3; 2.3 g./l. NaCl; and 1 mL/l. HNO3(42 B.).

3. A solution according to claim 1 having a pH between about 0.2 and 4.

4. A process for passivating and improving the corrosion resistance of zinc surfaces comprising subjecting the same to an aqueous acidic solution comprising 3 to 100 g./l. Na2C1'2O'z.2H2O; 1 to 35 g./l. HaBOs; 0.1 to 20 g./1. NaCl; and 1 to 10 m1./l. HNOa (42 B.), for

perature of about 70 F. to 200 F. and a pH of about 5. An aqueous acidic solution for passivating and improving c'orrosio-n resistance of zinc surfaces comprising 1) a member of the group consisting of chromic acid and water soluble salts thereof in an amount equivalent to 3 to 100 g./l. NazCrzOmZHzO; (2) a member of the group consisting of boric acid and water soluble salts thereof in an amount equivalent to 1 to 35 g./l. of HaBOa; (3) a member of the group consisting of hydrochloric acid and Water soluble salts thereof in an amount equivalent to 0.1 to 20 g./l. NaCl, and (4) a member of the group consisting of nitric acid and water soluble salts thereof in an amount equivalent to 1 to 10 ml./l. HNO3 (42 B.), said solution having a pH between about 0.2 and 4.

References Cited in the file of this patent UNITED STATES PATENTS Dennis Oct. 13, Wetherbee Oct. 29, Wilhelm Mar. 24, Wilhelm Feb. 1, Romig Apr. 12, Michel Nov. 1, Buzzard June 11, Ott Apr. 4,

OTHER REFERENCES Britton, Hydrogen Ions, page 286, Table 81. Goldowski, Abstract of application Serial No. 619,264, published April 29, 1952, 657 O. G. 1582. v 

5. AN AQUEOUS ACIDIC SOLUTION FOR PASSIVATING AND IMPROVING CORROSION RESISTANCE OF ZINC SURFACES COMPRISING (1) A MEMBER OF THE GROUP CONSISTING OF CHROMIC ACID AND WATER SOLUBLE SALTS THEREOF IN AN AMOUNT EQUIVALENT TO 3 TO 100 G./1. NA2CR2O72H2O; (2) A MEMBER OF THE GROUP CONSISTING OF BORIC ACID AND WATER SOLUBLE SALTS THEREOF IN AN AMOUNT EQUIVALENT TO 1 TO 35 G./1. OF H3BO3; (3) A MEMBER OF THE GROUP CONSISTING OF HYDROCHLORIC ACID AND WATER SOLUBLE SALTS THEREOF IN AN AMOUNT EQUIVALENT TO 0.1 TO 20 G./1. NAC1, AND (4) A MEMBER OF THE GROUP CONSISTING OF NITRIC ACID AND WATER SOLUBLE SALTS THEREOF IN AN AMOUNT EQUIVALENT TO 1 TO 10 ML./1. HNO3 (42*BE), SAID SOLUTION HAVING A PH BETWEEN ABOUT 0.2 AND
 4. 